Air-conditioning apparatus

ABSTRACT

An air-conditioning apparatus includes a compressor, an indoor heat exchanger, a first outdoor heat exchanger and a second outdoor heat exchanger, a valve between the second outdoor heat exchanger and the indoor heat exchanger switching between an opened state and a closed state, a temperature sensor detecting a temperature of refrigerant that flows into the second outdoor heat exchanger and liquefies, and a condensing-temperature detection device, wherein during first cooling operation in which the first outdoor heat exchanger functions as a condenser, the indoor heat exchanger functions as an evaporator, and the valve is in the closed state, when a comparative temperature becomes equal to or higher than a prescribed temperature, the air-conditioning apparatus switches to second cooling operation in which the valve is brought into the opened state, the comparative temperature being a value obtained by subtracting a temperature detected by the temperature sensor from the condensing temperature.

TECHNICAL FIELD

The present disclosure relates to an air-conditioning apparatus including a plurality of outdoor heat exchangers.

BACKGROUND ART

Hithereto, in a case where an indoor unit is installed in a computer room, for example, an air-conditioning apparatus may sometimes be required to perform cooling operation under the condition of low outside air temperature. For example, under the condition of low outside air temperature, the outside air temperature is equal to or lower than 0 degrees C. There have been related-art air-conditioning apparatuses, including an air-conditioning apparatus in which a plurality of outdoor heat exchangers are connected in parallel to a compressor (see, for example, Patent Literature 1). The air-conditioning apparatus, in which a plurality of outdoor heat exchangers are connected in parallel to a compressor, sometimes may run with operation of some of the outdoor heat exchangers being stopped depending on the running condition. For example, there is a case where the air-conditioning apparatus, in which a plurality of outdoor heat exchangers are connected in parallel to a compressor, performs cooling operation under the condition of low outside air temperature. In that case, the air-conditioning apparatus stops operation of some of the outdoor heat exchangers, while using the other outdoor heat exchangers as a condenser, such that the condensing temperature of refrigerant is maintained at a prescribed temperature or higher.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2011-052883

SUMMARY OF INVENTION Technical Problem

There is a case where an air-conditioning apparatus, in which a plurality of outdoor heat exchangers are connected in parallel to a compressor, performs cooling operation under the condition of low outside air temperature. In that case, refrigerant circulating in a refrigerant circuit may partially flow into a non-operating outdoor heat exchanger. The refrigerant flowing into the non-operating outdoor heat exchanger is liquefied by removal of heat from the refrigerant by the outside air. The liquefied refrigerant is thus accumulated in the non-operating outdoor heat exchanger. That is, the refrigerant is brought into a state of commonly called “stagnating” in the non-operating outdoor heat exchanger. As the refrigerant stagnates in the non-operating outdoor heat exchanger, a decreased amount of refrigerant circulates in the refrigerant circuit. Thus, as an increased amount of refrigerant stagnates in the non-operating outdoor heat exchanger, this prevents a sufficient amount of refrigerant from circulating in the refrigerant circuit, and consequently prevents the air-conditioning apparatus from exhibiting its desired cooling capacity.

For this reason, when the air-conditioning apparatus, in which a plurality of outdoor heat exchangers are connected in parallel to a compressor, is used to perform cooling operation under the condition of low outside air temperature, then a mechanism is needed to return the refrigerant stagnating in the non-operating outdoor heat exchanger to a refrigerant circulating flow passage. For example, the air-conditioning apparatus needs to be provided with an additional bypass pipe that connects the location where the refrigerant stagnates with the refrigerant circulating flow passage. Through this bypass pipe, the stagnating refrigerant can return to the refrigerant circulating flow passage. For another example, the air-conditioning apparatus needs to be provided with an additional bypass pipe that connects the location where the refrigerant stagnates with the discharge side of the compressor. Through this bypass pipe, the stagnating refrigerant can be evaporated by high-temperature refrigerant in gas form discharged from the compressor, and thus this stagnating refrigerant can return to the refrigerant circulating flow passage.

However, when the air-conditioning apparatus is provided with the additional bypass pipe as described above, this makes the refrigerant circuit more complicated. Further, when the air-conditioning apparatus is provided with the additional bypass pipe as described above, an outdoor unit needs to be increased in size to ensure an adequate installation space for the bypass pipe. For these reasons, when the related-art air-conditioning apparatus, in which a plurality of outdoor heat exchangers are connected in parallel to a compressor, is used to perform cooling operation under the condition of low outside air temperature, then a problem arises that the manufacturing costs of the air-conditioning apparatus increase.

The present disclosure has been achieved to solve the above problems, and it is an object of the present disclosure to provide an air-conditioning apparatus in which a plurality of outdoor heat exchangers are connected in parallel to a compressor, and in which an additional bypass pipe does not need to be provided even when the air-conditioning apparatus is used to perform cooling operation under the condition of low outside air temperature.

Solution to Problem

An air-conditioning apparatus according to an embodiment of the present disclosure includes: a compressor; an indoor heat exchanger; a first outdoor heat exchanger and a second outdoor heat exchanger connected in parallel between the compressor and the indoor heat exchanger; a valve provided between the second outdoor heat exchanger and the indoor heat exchanger to switch between an opened state in which a flow passage between the second outdoor heat exchanger and the indoor heat exchanger is opened, and a closed state in which the flow passage between the second outdoor heat exchanger and the indoor heat exchanger is dosed; a temperature sensor configured to detect a temperature of refrigerant that flows into the second outdoor heat exchanger and liquefies; and a condensing-temperature detection device configured to detect a condensing temperature of refrigerant, wherein during first cooling operation in which the first outdoor heat exchanger functions as a condenser, the indoor heat exchanger functions as an evaporator, and the valve is in the closed state, when a comparative temperature becomes equal to or higher than a prescribed temperature, the air-conditioning apparatus switches to second cooling operation in which the valve is brought into the opened state, the comparative temperature being a value obtained by subtracting a temperature detected by the temperature sensor from the condensing temperature.

Advantageous Effects of Invention

When refrigerant stagnates in a non-operating second outdoor heat exchanger during the first cooling operation, the air-conditioning apparatus according to an embodiment of the present disclosure switches the valve from the closed state to the opened state, and whereby the refrigerant stagnating in the second outdoor heat exchanger 20 can be returned to the refrigerant circulating flow passage. Due to this operation, the air-conditioning apparatus according to an embodiment of the present disclosure does not need to be provided with an additional bypass pipe through which the refrigerant stagnating in the non-operating outdoor heat exchanger can return to the refrigerant circulating flow passage.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a refrigerant circuit diagram of an air-conditioning apparatus according to Embodiment 1.

FIG. 2 is a flowchart describing operation of the air-conditioning apparatus according to Embodiment 1 during a cooling period.

FIG. 3 is a flowchart describing another operation of the air-conditioning apparatus according to Embodiment 1 during a cooling period.

FIG. 4 is an explanatory diagram describing the method for determining the airflow amount of an outdoor fan in the air-conditioning apparatus according to Embodiment 1.

FIG. 5 is a refrigerant circuit diagram of an air-conditioning apparatus according to Embodiment 2.

FIG. 6 is a flowchart describing operation of the air-conditioning apparatus according to Embodiment 2 during a cooling period.

DESCRIPTION OF EMBODIMENTS Embodiment 1

FIG. 1 is a refrigerant circuit diagram of an air-conditioning apparatus according to Embodiment 1.

An air-conditioning apparatus 100 is capable of performing at least cooling operation. The air-conditioning apparatus 100 includes a compressor 1, an indoor heat exchanger 3, a first outdoor heat exchanger 10, a second outdoor heat exchanger 20, a valve 22, a temperature sensor 51, and a condensing-temperature detection device 30. The first outdoor heat exchanger 10 and the second outdoor heat exchanger 20 are connected in parallel between the compressor 1 and the indoor heat exchanger 3.

The compressor 1 compresses refrigerant. The compressor 1 includes a suction port 1 b through which the compressor 1 sucks refrigerant to be compressed, and a discharge port 1 a through which the compressor 1 discharges the compressed refrigerant. The compressor 1 can be made up of, for example, a rotary compressor, a scroll compressor, a screw compressor, or a reciprocating compressor. Note that the air-conditioning apparatus 100 according to Embodiment 1 includes a check valve 2 to prevent backflow of refrigerant into the compressor 1 through the discharge port 1 a. The air-conditioning apparatus 100 according to Embodiment 1 further includes an accumulator 7 on the refrigerant suction side of the compressor 1 to accumulate surplus refrigerant therein. The compressor 1 is thus configured to suck refrigerant, having once flowed into the accumulator 7, from the suction port 1 b. Some of the constituents of the air-conditioning apparatus 100 communicate with the suction port 1 b of the compressor 1 through the accumulator 7.

The indoor heat exchanger 3 functions as an evaporator during cooling operation. The indoor heat exchanger 3 can be made up of, for example, a fin-and-tube heat exchanger, a microchannel heat exchanger, a shell-and-tube heat exchanger, a heat-pipe heat exchanger, a double-pipe heat exchanger, or a plate heat exchanger. A flow control valve 4 is provided on the pipe section at a position upstream of the indoor heat exchanger 3 in the refrigerant flow direction when the indoor heat exchanger 3 functions as an evaporator. The flow control valve 4 regulates the flow rate of refrigerant that flows through the indoor heat exchanger 3. The air-conditioning apparatus 100 according to Embodiment 1 is configured to be capable of also performing heating operation. The indoor heat exchanger 3 functions as a condenser during heating operation. Note that the air-conditioning apparatus 100 may include a plurality of indoor heat exchangers 3. In this case, for example, a set of the indoor heat exchanger 3 and the flow control valve 4 is arranged in parallel with another set of the indoor heat exchanger 3 and the flow control valve 4.

The first outdoor heat exchanger 10 communicates with the discharge port 1 a of the compressor 1, and functions as a condenser during cooling operation. The first outdoor heat exchanger 10 can be made up of, for example, a fin-and-tube heat exchanger, a microchannel heat exchanger, a shell-and-tube heat exchanger, a heat-pipe heat exchanger, a double-pipe heat exchanger, or a plate heat exchanger.

Note that, as described above, the air-conditioning apparatus 100 according to Embodiment 1 is configured to be capable of also performing heating operation. Due to this configuration, the air-conditioning apparatus 100 includes a flow switching device 11 between the first outdoor heat exchanger 10 and the discharge port 1 a of the compressor 1. The flow switching device 11 switches the communication destination for the discharge port 1 a of the compressor 1 to the indoor heat exchanger 3 or to the first outdoor heat exchanger 10. Specifically, the flow switching device 11 allows the discharge port 1 a of the compressor 1 to communicate with the first outdoor heat exchanger 10, while allowing the suction port 1 b of the compressor 1 to communicate with the indoor heat exchanger 3 during cooling operation. The flow switching device 11 also allows the discharge port 1 a of the compressor 1 to communicate with the indoor heat exchanger 3, while allowing the suction port 1 b of the compressor 1 to communicate with the first outdoor heat exchanger 10 during heating operation. Note that in Embodiment 1, the flow switching device 11 is made up of a four-way valve, however, the flow switching device 11 may also be made up of a two-way valve or other type of valve. In a case where the air-conditioning apparatus 100 is configured to perform only cooling operation, the flow switching device 11 is not needed.

The air-conditioning apparatus 100 according to Embodiment 1 is not only presumed to cause the first outdoor heat exchanger 10 to function as a condenser or an evaporator all the time, but is also presumed to run while operation of the first outdoor heat exchanger 10 stopped. Based on this presumption, a valve 12 is provided between the first outdoor heat exchanger 10 and the indoor heat exchanger 3. In Embodiment 1, the valve 12 is an opening-closing valve. The valve 12 switches between an opened state and a closed state. In the opened state, the flow passage between the first outdoor heat exchanger 10 and the indoor heat exchanger 3 is opened. In the closed state, the flow passage between the first outdoor heat exchanger 10 and the indoor heat exchanger 3 is closed. As the valve 12 is brought into the closed state, the first outdoor heat exchanger 10 is brought into a non-operating state.

The second outdoor heat exchanger 20 communicates with the discharge port 1 a of the compressor 1, and functions as a condenser during cooling operation. The second outdoor heat exchanger 20 can be made up of, for example, a fin-and-tube heat exchanger, a microchannel heat exchanger, a shell-and-tube heat exchanger, a heat-pipe heat exchanger, a double-pipe heat exchanger, or a plate heat exchanger.

Note that, as described above, the air-conditioning apparatus 100 according to Embodiment 1 is configured to be capable of also performing heating operation. Due to this configuration, the air-conditioning apparatus 100 includes a flow switching device 21 between the second outdoor heat exchanger 20 and the discharge port 1 a of the compressor 1. The flow switching device 21 switches between a first flow passage state and a second flow passage state. In the first flow passage state, the discharge port 1 a of the compressor 1 communicates with the second outdoor heat exchanger 20. In the second flow passage state, the discharge port 1 a of the compressor I does not communicate with the second outdoor heat exchanger 20. In FIG. 1, the flow passage in a state illustrated by the dotted lines is defined as the first flow passage state. In addition, the flow passage in a state illustrated by the solid lines is defined as the second flow passage state. As described above, the air-conditioning apparatus 100 according to Embodiment 1 is configured to be capable of also performing heating operation. Due to this configuration, in the second flow passage state, the flow switching device 21 according to Embodiment 1 allows the suction port 1 b of the compressor I to communicate with the second outdoor heat exchanger 20, such that the second outdoor heat exchanger 20 functions as an evaporator in the second flow passage state.

More specifically, when the flow switching device 21 is in the first flow passage state, the flow switching device 21 allows the discharge port 1 a of the compressor 1 to communicate with the second outdoor heat exchanger 20, while allowing the suction port 1 b of the compressor 1 to communicate with the indoor heat exchanger 3. Thus, the flow switching device 21 is brought into the first flow passage state during cooling operation, so that the second outdoor heat exchanger 20 functions as a condenser. When the flow switching device 21 is in the second flow passage state, the flow switching device 21 allows the discharge port 1 a of the compressor 1 to communicate with the indoor heat exchanger 3, while allowing the suction port 1 b of the compressor 1 to communicate with the second outdoor heat exchanger 20. Thus, the flow switching device 21 is brought into the second flow passage state during heating operation, so that the second outdoor heat exchanger 20 functions as an evaporator. Note that in Embodiment 1, the flow switching device 21 is made up of a four-way valve, however, the flow switching device 21 may also be made up of a two-way valve or other type of valve. In a case where the air-conditioning apparatus 100 is configured to perform only cooling operation, the flow switching device 21 is not needed.

In Embodiment 1, the valve 22 is an opening-closing valve. The valve 22 switches between an opened state and a closed state. In the opened state, the flow passage between the second outdoor heat exchanger 20 and the indoor heat exchanger 3 is opened. In the closed state, the flow passage between the second outdoor heat exchanger 20 and the indoor heat exchanger 3 is closed. As the valve 22 is brought into the closed state during cooling operation, the second outdoor heat exchanger 20 is brought into a non-operating state. Note that in Embodiment 1, the flow switching device 21 is configured to be in the second flow passage state when the second outdoor heat exchanger 20 is brought into a non-operating state during cooling operation.

Note that a flow control valve 5 is provided on the pipe section between the first outdoor heat exchanger 10 and the indoor heat exchanger 3, and between the second outdoor heat exchanger 20 and the indoor heat exchanger 3, and the flow control valve 5 regulates the flow rate of refrigerant that flows through the first outdoor heat exchanger 10 and the second outdoor heat exchanger 20. In a case where a single indoor heat exchanger 3 is provided, the flow rate of refrigerant is regulated by the flow control valve 4 or the flow control valve 5, and whereby the flow rate of refrigerant that flows through the first outdoor heat exchanger 10 and the second outdoor heat exchanger 20 can be regulated, and the flow rate of refrigerant that flows through the indoor heat exchanger 3 can also be regulated. Due to this configuration, in the case where a single indoor heat exchanger 3 is provided, only one of the flow control valve 4 and the flow control valve 5 may be provided.

The air-conditioning apparatus 100 according to Embodiment 1 further includes an outdoor fan 6 to supply outside air to the first outdoor heat exchanger 10 and the second outdoor heat exchanger 20, Note that in Embodiment 1, the outdoor fan 6 is made up of a single fan to supply outside air to both the first outdoor heat exchanger 10 and the second outdoor heat exchanger 20. The outdoor fan 6 is not limited to being made up of a single fan, but may also be made up of a fan to supply outside air to the first outdoor heat exchanger 10 and another fan to supply outside air to the second outdoor heat exchanger 20. That is, the outdoor fan 6 may be made up of a plurality of fans. In this case, the airflow amount of the outdoor fan 6 indicates the total airflow amount of the plurality of fans.

The air-conditioning apparatus 100 according to Embodiment 1 further includes a subcooling heat exchanger 41, a bypass pipe 42, and a flow control valve 43. The subcooling heat exchanger 41 is provided on the pipe section between the first outdoor heat exchanger 10 and the flow control valve 5, and between the second outdoor heat exchanger 20 and the flow control valve 5. One end portion of the bypass pipe 42 is connected to the pipe section between the subcooling heat exchanger 41 and the flow control valve 5. The other end portion of the bypass pipe 42 is connected to the inflow-side pipe of the accumulator 7. An intermediate portion of the bypass pipe 42 is located inside the subcooling heat exchanger 41. The flow control valve 43 is provided in the bypass pipe 42 at a position upstream of the subcooling heat exchanger 41 in the flow direction of refrigerant flowing through the bypass pipe 42.

During cooling operation, refrigerant in liquid form, having flowed out of a heat exchanger that is either the first outdoor heat exchanger 10 or the second outdoor heat exchanger 20. whichever functions as a condenser, passes through the subcooling heat exchanger 41. The refrigerant in liquid form, having passed through the subcooling heat exchanger 41, partially flows into the bypass pipe 42, then is decompressed by the flow control valve 43 to a comparatively low temperature, and then passes through the subcooling heat exchanger 41 again. Due to this flow, during cooling operation, the refrigerant in liquid form, having flowed out of a heat exchanger that is either the first outdoor heat exchanger 10 or the second outdoor heat exchanger 20, whichever functions as a condenser, is cooled by refrigerant having flowed into the bypass pipe 42. Therefore, during cooling operation, the degree of subcooling of the refrigerant flowing into the indoor heat exchanger 3 can be increased, and accordingly cooling performance of the air-conditioning apparatus 100 can be improved.

The temperature sensor 51 detects a temperature of refrigerant that flows into the second outdoor heat exchanger 20 and liquefies. Specifically, when the air-conditioning apparatus 100 performs first cooling operation, as will be described later, under the condition of low outside air temperature, refrigerant circulating in the refrigerant circuit may partially flow into the second outdoor heat exchanger 20 in a non-operating state. For example, under the condition of low outside air temperature, the outside air temperature is equal to or lower than 0 degrees C. The refrigerant flowing into the second outdoor heat exchanger 20 in a non-operating state is liquefied by removal of heat from the refrigerant by the outside air. The liquefied refrigerant is then accumulated in the non-operating second outdoor heat exchanger 20. That is, the refrigerant is brought into a state of commonly called “stagnating” in the second outdoor heat exchanger 20 in a non-operating state. The temperature sensor 51 detects a temperature of this stagnating refrigerant.

Note that in the configuration of the air-conditioning apparatus 100 according to Embodiment 1, refrigerant stagnating in the second outdoor heat exchanger 20 flows toward the valve 22. Due to this configuration, in Embodiment 1, the temperature sensor 51 is provided on the pipe section between the second outdoor heat exchanger 20 and the valve 22. However, this installation position of the temperature sensor 51 is shown merely as an example. The temperature sensor 51 can be installed at any position, provided that the temperature sensor 51 can detect a temperature of refrigerant stagnating in the second outdoor heat exchanger 20. For example, the temperature sensor 51 may be provided on the lower portion of the second outdoor heat exchanger 20. The air-conditioning apparatus 100 according to Embodiment 1 is also provided with a temperature sensor 52 to detect an outside air temperature.

The condensing-temperature detection device 30 detects a condensing temperature of refrigerant circulating in the refrigerant circuit of the air-conditioning apparatus 100. In Embodiment 1, the condensing-temperature detection device 30 includes a pressure sensor 31 to detect a pressure of refrigerant discharged from the compressor 1, and a computation unit 61 to compute a condensing temperature of refrigerant circulating in the refrigerant circuit of the air-conditioning apparatus 100 based on the pressure detected by the pressure sensor 31. In Embodiment 1, the computation unit 61 is configured to be a functional unit of a controller 60 that will be described later. Note that this configuration of the condensing-temperature detection device 30 is shown merely as an example. Various configurations to calculate the condensing temperature of refrigerant circulating in the refrigerant circuit have heretofore been proposed. Any of these related-art configurations may be employed as the condensing-temperature detection device 30.

Note that, hitherto, an air-conditioning apparatus is provided with sensors such as a temperature sensor and a pressure sensor at various positions to control the running state, It is also allowable that the air-conditioning apparatus 100 according to Embodiment 1 is provided with a sensor to control the running state, other than the sensors described above.

As described above, the air-conditioning apparatus 100 according to Embodiment 1 includes the controller 60. Based on the signals detected by various types of sensors provided in the air-conditioning apparatus 100 and other signals, the controller 60 controls the rotation speed of the compressor 1, the opening degree of the flow control valve 4, the opening degree of the flow control valve 5, the rotation speed of the outdoor fan 6, the flow passage of the flow switching device 11, opening and closing of the valve 12, the flow passage of the flow switching device 21, opening and closing of the valve 22, the opening degree of the flow control valve 43, and other conditions. The controller 60 is made up of either dedicated hardware or a central processing unit (CPU) to execute programs stored in a memory. Note that the CPU is also referred to as “central processing device,” “processing device,” “computation device,” “microprocessor,” “microcomputer,” or “processor.”

When the controller 60 is dedicated hardware, the controller 60 is equivalent to, for example, a single circuit, a combined circuit, an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or a combination thereof. The functional units of the controller 60 may be individually implemented by separate units of hardware, or may be implemented together by a single unit of hardware.

When the controller 60 is the CPU, the functions to be executed by the controller 60 are implemented by software, firmware, or a combination of the software and the firmware. The software and firmware are described as programs and stored in the memory. The CPU reads and executes the programs stored in the memory, thereby implementing the functions of the controller 60. For example, the memory is a nonvolatile or volatile semiconductor memory such as a RAM, a ROM, a flash memory, an EPROM, or an EEPROM.

Note that the functions of the controller 60 may be partially implemented by dedicated hardware, while being partially implemented by software or firmware.

The controller 60 includes, as its functional units, the computation unit 61, a control unit 62, a fan airflow-amount determination unit 63, and a storage unit 65. The computation unit 61 is a functional unit to compute various numerical values to be used for various controls executed by the control unit 62. For example, the computation unit 61 computes a condensing temperature of refrigerant circulating in the refrigerant circuit of the air-conditioning apparatus 100 based on the pressure detected by the pressure sensor 31, as described above. For another example, the computation unit 61 computes a comparative temperature that is a value obtained by subtracting the temperature detected by the temperature sensor 51 from the condensing temperature of refrigerant circulating in the refrigerant circuit. The control unit 62 is a functional unit configured to control the rotation speed of the compressor 1, the opening degree of the flow control valve 4, the opening degree of the flow control valve 5, the rotation speed of the outdoor fan 6, the flow passage of the flow switching device 11, opening and closing of the valve 12, the flow passage of the flow switching device 21, opening and closing of the valve 22, the opening degree of the flow control valve 43, and other conditions. The fan airflow-amount determination unit 63 is a functional unit to determine the airflow amount of the outdoor fan 6. The storage unit 65 is a functional unit to store therein various types of data necessary for controlling the air-conditioning apparatus 100.

The constituents of the air-conditioning apparatus 100 described above are accommodated either in an outdoor unit 101 or an indoor unit 102. In Embodiment 1, the outdoor unit 101 has accommodated therein the compressor 1, the check valve 2, the flow control valve 5, the outdoor fan 6, the accumulator 7, the first outdoor heat exchanger 10, the flow switching device 11, the valve 12, the second outdoor heat exchanger 20, the flow switching device 21, the valve 22, the pressure sensor 31, the subcooling heat exchanger 41, the bypass pipe 42, the flow control valve 43, the temperature sensor 51, the temperature sensor 52, and the controller 60. The indoor unit 102 has the indoor heat exchanger 3 and the flow control valve 4 accommodated therein.

The air-conditioning apparatus 100 according to Embodiment 1 further includes an opening-closing valve 8 and an opening-closing valve 9. A section of the refrigerant circuit, located in the outdoor unit 101, and another section of the refrigerant circuit, located in the indoor unit 102, can be connected to, and disconnected from, each other by the opening-closing valve 8 and the opening-closing valve 9. For example, the outdoor unit 101 is installed at its installation location in a state in which a section of the refrigerant circuit, located in the outdoor unit 101, is filled with refrigerant, and the opening-closing valve 8 and the opening-closing valve 9 are both closed. The indoor unit 102 is also installed at its installation location. Thereafter, a section of the refrigerant circuit, located in the outdoor unit 101, and another section of the refrigerant circuit, located in the indoor unit 102, are connected to each other by the opening-closing valve 8 and the opening-closing valve 9, and then the opening-closing valve 8 and the opening-closing valve 9 are both opened. This allows refrigerant to circulate in the refrigerant circuit of the air-conditioning apparatus 100.

Subsequently, operation of the air-conditioning apparatus 100 is described. Note that cooling operation performed by the air-conditioning apparatus 100 under the condition of low outside air temperature is described below. When performing cooling operation under the condition of low outside air temperature, the air-conditioning apparatus 100 performs first cooling operation to maintain the condensing temperature of refrigerant at a prescribed temperature or higher. For example, when the temperature sensor 52 detects the outside air temperature equal to or lower than 0 degrees C., the air-conditioning apparatus 100 performs the first cooling operation. During the first cooling operation, the first outdoor heat exchanger 10 functions as a condenser, while the indoor heat exchanger 3 functions as an evaporator. During the first cooling operation, the valve 22 is brought into the closed state. That is, the second outdoor heat exchanger 20 is brought into a non-operating state. During the first cooling operation, refrigerant circulates in the refrigerant circuit of the air-conditioning apparatus 100 in the following manner.

High-temperature and high-pressure refrigerant in gas form compressed in the compressor 1 and discharged from the discharge port 1 a of the compressor 1 passes through the flow switching device 11 and flows into the first outdoor heat exchanger 10. The high-temperature and high-pressure refrigerant in gas form, having flowed into the first outdoor heat exchanger 10, is cooled into high-pressure refrigerant in liquid form by the outside air supplied by the outdoor fan 6. Then, the high-pressure refrigerant in liquid form flows out of the first outdoor heat exchanger 10. The high-pressure refrigerant in liquid form, having flowed out of the first outdoor heat exchanger 10, passes through the valve 12, and then flows into the subcooling heat exchanger 41. The high-pressure refrigerant in liquid form, having flowed into the subcooling heat exchanger 41, is cooled by refrigerant flowing through the bypass pipe 42, then increases its degree of subcooling, and flows out of the subcooling heat exchanger 41.

The high-pressure refrigerant in liquid form, having flowed out of the subcooling heat exchanger 41, is decompressed into low-temperature and low-pressure two-phase gas-liquid refrigerant by at least one of the flow control valve 5 and the flow control valve 4. Then, the low-temperature and low-pressure two-phase gas-liquid refrigerant flows into the indoor heat exchanger 3. This low-temperature and low-pressure two-phase gas-liquid refrigerant, having flowed into the indoor heat exchanger 3, cools the room air and evaporates into low-pressure refrigerant in gas form. Then, the low-pressure refrigerant in gas form flows out of the indoor heat exchanger 3. This low-pressure refrigerant in gas form, having flowed out of the indoor heat exchanger 3, passes through the flow switching device 11 and the accumulator 7, is suctioned into the compressor 1 from the suction port 1 b of the compressor 1, and is compressed again in the compressor 1.

During a period for which the air-conditioning apparatus 100 performs the first cooling operation described above under the condition of low outside air temperature, refrigerant circulating in the refrigerant circuit may partially flow into the second outdoor heat exchanger 20 in a non-operating state. In Embodiment 1, low-pressure refrigerant in gas form, having flowed out of the indoor heat exchanger 3, may sometimes pass through the flow switching device 21, and flow into the second outdoor heat exchanger 20 in a non-operating state. Note that there is a case where the flow switching device 21 is configured to be in the first flow passage state when the second outdoor heat exchanger 20 is brought into a non-operating state during cooling operation, and in that case, high-temperature and high-pressure refrigerant in gas form, discharged from the discharge port 1 a of the compressor 1, may sometimes flow into the second outdoor heat exchanger 20 in a non-operating state. There is also a case where the air-conditioning apparatus 100 is configured to only perform cooling operation, and is thus not provided with the flow switching device 21. In that case, the high-temperature and high-pressure refrigerant in gas form, discharged from the discharge port 1 a of the compressor 1, may sometimes flow into the second outdoor heat exchanger 20 in a non-operating state. It is thus conceivable to provide an opening-closing valve at the position of the flow switching device 21 to open or close the flow passage between the second outdoor heat exchanger 20 and the discharge port 1 a of the compressor 1. It is also conceivable to close the flow passage between the second outdoor heat exchanger 20 and the discharge port 1 a of the compressor 1 by using the opening-closing valve when the second outdoor heat exchanger 20 is brought into a non-operating state during cooling operation. However, even though the air-conditioning apparatus 100 is configured as described above, high-temperature and high-pressure refrigerant in gas form, discharged from the discharge port 1 a of the compressor 1, may still pass through a slight gap of the opening-closing valve and flow into the second outdoor heat exchanger 20 in a non-operating state.

The refrigerant having flowed into the second outdoor heat exchanger 20 in a non-operating state is liquefied by removal of heat from the refrigerant by the outside air. The liquefied refrigerant then stagnates in the non-operating second outdoor heat exchanger 20. As the refrigerant stagnates in the second outdoor heat exchanger 20 in a non-operating state, a decreased amount of refrigerant circulates in the refrigerant circuit. Thus, as an increased amount of refrigerant stagnates in the second outdoor heat exchanger 20 in a non-operating state, a sufficient amount of refrigerant is prevented from circulating in the refrigerant circuit, and consequently the air-conditioning apparatus 100 is prevented from exhibiting its desired cooling capacity. To cope with this problem, the air-conditioning apparatus 100 according to Embodiment 1 performs the following operation during a cooling period to prevent an excessive amount of refrigerant from stagnating in the second outdoor heat exchanger 20 in a non-operating state.

FIG. 2 is a flowchart describing operation of the air-conditioning apparatus according to Embodiment 1 during a cooling period. When cooling operation of the air-conditioning apparatus 100 is started, the controller 60 starts the operation illustrated in FIG. 2 in step S1. The controller 60 continues the operation illustrated in FIG. 2 until cooling operation of the air-conditioning apparatus 100 is stopped. After step S1, the controller 60 determines whether operation of the second outdoor heat exchanger 20 is stopped in step S2. That is, the controller 60 determines whether the air-conditioning apparatus 100 performs first cooling operation in step S2.

When operation of the second outdoor heat exchanger 20 is not determined to be stopped in step S2, the controller 60 repeats the operation in step S2 until operation of the second outdoor heat exchanger 20 is stopped. In contrast, when operation of the second outdoor heat exchanger 20 is determined to be stopped in step S2, the computation unit 61 in the controller 60 computes a comparative temperature in step S3. As described above, the comparative temperature is a value obtained by subtracting the temperature detected by the temperature sensor 51 from the condensing temperature of refrigerant circulating in the refrigerant circuit. The condensing temperature of refrigerant circulating in the refrigerant circuit is detected by the condensing-temperature detection device 30.

After step S3, the control unit 62 in the controller 60 determines whether the comparative temperature is equal to or higher than a prescribed temperature in step S4. The prescribed temperature is stored in the storage unit 65 in the controller 60 in advance.

The determination of whether the comparative temperature is equal to or higher than the prescribed temperature is performed to estimate the amount of refrigerant stagnating in the non-operating second outdoor heat exchanger 20. Specifically, when the air-conditioning apparatus 100 performs cooling operation under the condition of low outside air temperature, the temperature of refrigerant that flows into the non-operating second outdoor heat exchanger 20 is higher than the outside air temperature. The refrigerant, having flowed into the non-operating second outdoor heat exchanger 20, is cooled by the outside air, so that the temperature of the refrigerant decreases as the time elapses. That is, the temperature of the refrigerant detected by the temperature sensor 51 decreases as the time elapses. Therefore, the comparative temperature increases as the time elapses. The comparative temperature is a value obtained by subtracting the temperature detected by the temperature sensor 51 from the condensing temperature of refrigerant circulating in the refrigerant circuit.

While a sufficient amount of refrigerant circulates in the refrigerant circuit, the condensing temperature of refrigerant circulating in the refrigerant circuit becomes stable during the first cooling operation. While a sufficient amount of refrigerant circulates in the refrigerant circuit, an evaporating temperature of refrigerant circulating in the refrigerant circuit also becomes stable during the first cooling operation. That is, while a sufficient amount of refrigerant circulates in the refrigerant circuit, a pressure of the refrigerant circulating on a high-pressure side in the refrigerant circuit and a pressure of the refrigerant circulating on a low-pressure side in the refrigerant circuit both become stable during the first cooling operation. Thus, while a sufficient amount of refrigerant circulates in the refrigerant circuit, the temperature of refrigerant that flows into the non-operating second outdoor heat exchanger 20 also becomes stable during the first cooling operation. The amount of refrigerant that flows into the non-operating second outdoor heat exchanger 20 per unit time also becomes stable. Thus, the amount of refrigerant stagnating in the non-operating second outdoor heat exchanger 20 can be estimated from the comparative temperature.

Based on this concept, when the comparative temperature is not equal to or higher than the prescribed temperature, it can be determined that the amount of refrigerant stagnating in the non-operating second outdoor heat exchanger 20 is still relatively small. Thus, when the comparative temperature is not equal to or higher than the prescribed temperature in step S4, the controller 60 repeats step S3 and step S4 until the comparative temperature becomes equal to or higher than the prescribed temperature in step S4. In contrast, when the comparative temperature is equal to or higher than the prescribed temperature, it can be determined that a certain amount or more of refrigerant stagnates in the non-operating second outdoor heat exchanger 20. For this reason, when the comparative temperature is equal to or higher than the prescribed temperature in step S4, the controller 60 performs operation from step S5 to step S7 to return the refrigerant stagnating in the second outdoor heat exchanger 20 to the refrigerant circulating flow passage. That is, the refrigerant stagnating in the second outdoor heat exchanger 20 is recovered.

In step S5, the control unit 62 brings the valve 22 into the opened state. In other words, when the comparative temperature becomes equal to or higher than the prescribed temperature during the first cooling operation, the air-conditioning apparatus 100 switches the first cooling operation to second cooling operation in which the valve 22 is brought into the opened state. Note that as described above, the air-conditioning apparatus 100 according to Embodiment 1 includes the flow switching device 21. Due to this configuration, in Embodiment 1, the control unit 62 brings the flow switching device 21 into the first flow passage state in step S5. With this operation, high-temperature and high-pressure refrigerant in gas form, discharged from the discharge port 1 a of the compressor 1, flows into the second outdoor heat exchanger 20. Then, the refrigerant stagnating in the second outdoor heat exchanger 20 is heated by the high-temperature and high-pressure refrigerant in gas form flowing into the second outdoor heat exchanger 20, and evaporates, and then flows out of the second outdoor heat exchanger 20. This allows the refrigerant stagnating in the second outdoor heat exchanger 20 to return to the refrigerant circulating flow passage.

After step S5, the computation unit 61 computes a comparative temperature in step S6. Then, in step 37, the control unit 62 determines whether the comparative temperature is lower than the prescribed temperature. When the comparative temperature is not lower than the prescribed temperature, it can be determined that the refrigerant stagnating in the second outdoor heat exchanger 20 has not yet returned sufficiently to the refrigerant circulating flow passage. For this reason, when the comparative temperature is not lower than the prescribed temperature, the controller 60 repeats step S6 and step S7 until the comparative temperature becomes lower than the prescribed temperature.

In contrast, when the comparative temperature is lower than the prescribed temperature, it can be determined that the refrigerant stagnating in the second outdoor heat exchanger 20 has sufficiently returned to the refrigerant circulating flow passage. For this reason, when the comparative temperature is lower than the prescribed temperature in step S7, the control unit 62 brings the valve 22 into the closed state in step S8. That is, the control unit 62 causes the air-conditioning apparatus 100 to return to the first cooling operation. Note that as described above, the air-conditioning apparatus 100 according to Embodiment 1 includes the flow switching device 21. Due to this configuration, in Embodiment 1, the control unit 62 brings the flow switching device 21 into the second flow passage state in step S8. Thereafter, the controller 60 returns to step S2.

When performing the cooling operation under the condition of low outside air temperature, the air-conditioning apparatus 100 operates in the manner as described above. Thus, the air-conditioning apparatus 100 can return the refrigerant stagnating in the second outdoor heat exchanger 20 to the refrigerant circulating flow passage without the need for providing an additional bypass pipe, unlike the related-art air-conditioning apparatus. Due to this configuration, even when the air-conditioning apparatus 100 is used to perform cooling operation under the condition of low outside air temperature, the increase in manufacturing costs can still be minimized compared to the related-art air-conditioning apparatus.

Note that when performing cooling operation under the condition of low outside air temperature, the air-conditioning apparatus 100 may operate in a manner as illustrated in FIG. 3.

FIG. 3 is a flowchart describing another operation of the air-conditioning apparatus according to Embodiment 1 during a cooling period.

As illustrated in FIG. 3, the air-conditioning apparatus 100 performs the operation in step S11 and step S12 in place of the operation in step S5 illustrated in FIG. 2. Specifically, when the comparative temperature is equal to or higher than the prescribed temperature in step S4, the fan airflow-amount determination unit 63 determines the airflow amount of the outdoor fan 6 in step S11. More specifically, the fan airflow—amount determination unit 63 determines a reduced airflow amount of the outdoor fan 6 relative to the present airflow amount of the outdoor fan 6.

After step S11, the control unit 62 brings the valve 22 into the opened state in step S12 similarly to step S5 in FIG. 2 to switch the first cooling operation to the second cooling operation. In step S12, the control unit 62 brings the flow switching device 21 into the first flow passage state similarly to step S5 in FIG. 2. With this operation, high-temperature and high-pressure refrigerant in gas form, discharged from the discharge port 1 a of the compressor 1, flows into the second outdoor heat exchanger 20. Then, the refrigerant stagnating in the second outdoor heat exchanger 20 is heated by the high-temperature and high-pressure refrigerant in gas form flowing into the second outdoor heat exchanger 20, and evaporates, and then flows out of the second outdoor heat exchanger 20. This allows the refrigerant stagnating in the second outdoor heat exchanger 20 to return to the refrigerant circulating flow passage,

Further, in step S12, the control unit 62 decreases the airflow amount of the outdoor fan 6. That is, the airflow amount of the outdoor fan 6 during the second cooling operation is reduced relative to the airflow amount of the outdoor fan 6 during the first cooling operation.

During the first cooling operation, only the first outdoor heat exchanger 10 functions as a condenser. Due to this function, during the first cooling operation, the heat exchange capacity of the condenser in the air-conditioning apparatus 100 is defined as a heat exchange capacity of the first outdoor heat exchanger 10. In contrast, during the second cooling operation, both the first outdoor heat exchanger 10 and the second outdoor heat exchanger 20 function as a condenser. Due to this function, during the second cooling operation, the heat exchange capacity of the condenser in the air-conditioning apparatus 100 is defined as the total heat exchange capacity of the first outdoor heat exchanger 10 and the second outdoor heat exchanger 20. Therefore, in a case where the airflow amount of the outdoor fan 6 during the first cooling operation is equal to that during the second cooling operation, when the first cooling operation is switched to the second cooling operation to recover the refrigerant stagnating in the second outdoor heat exchanger 20, then the heat exchange capacity of the condenser in the air-conditioning apparatus 100 increases significantly,

Accordingly, when the first cooling operation is switched to the second cooling operation to recover the refrigerant stagnating in the second outdoor heat exchanger 20, then there are significant variations in the pressure of refrigerant circulating on a high-pressure side in the refrigerant circuit and refrigerant circulating on a low-pressure side in the refrigerant circuit. Consequently, when the first cooling operation is switched to the second cooling operation to recover the refrigerant stagnating in the second outdoor heat exchanger 20, then there are significant variations in the temperature of air blown from the indoor unit 102 toward an air-conditioned space.

On the other hand, at the time of switching the first cooling operation to the second cooling operation, the airflow amount of the outdoor fan 6 is reduced, so that there are insignificant variations in the heat exchange capacity of the condenser in the air-conditioning apparatus 100 between the first cooling operation and the second cooling operation. Accordingly, at the time of switching the first cooling operation to the second cooling operation, the airflow amount of the outdoor fan 6 is reduced, so that the variations in the pressure of refrigerant circulating on a high-pressure side in the refrigerant circuit and refrigerant circulating on a low-pressure side in the refrigerant circuit can be reduced between the first cooling operation and the second cooling operation. Consequently, at the time of switching the first cooling operation to the second cooling operation, the airflow amount of the outdoor fan 6 is reduced, so that the variations in the temperature of air blown from the indoor unit 102 toward an air-conditioned space can be reduced between the first cooling operation and the second cooling operation.

Note that in Embodiment 1, the airflow amount of the outdoor fan 6 at the time of switching the first cooling operation to the second cooling operation is determined in step S11 in the following manner.

FIG. 4 is an explanatory diagram describing the method for determining the airflow amount of an outdoor fan in the air-conditioning apparatus according to Embodiment 1, The horizontal axis in FIG. 4 illustrates the heat exchange capacity of the condenser. FIG. 4 shows that the heat exchange capacity of the condenser increases toward the right side of the horizontal axis. The vertical axis in FIG. 4 illustrates the airflow amount of the outdoor fan 6. FIG. 4 shows that the airflow amount of the outdoor fan 6 increases toward the upper side of the vertical axis. The solid line A illustrated in FIG. 4 shows the relationship between the heat exchange capacity of the condenser and the airflow amount of the outdoor fan 6 when the air-conditioning apparatus 100 performs the first cooling operation. That is, the solid line A illustrated in FIG. 4 shows the relationship between the heat exchange capacity of the first outdoor heat exchanger 10 and the airflow amount of the outdoor fan 6. The solid line B illustrated in FIG. 4 shows the relationship between the heat exchange capacity of the condenser and the airflow amount of the outdoor fan 6 when the air-conditioning apparatus 100 performs the second cooling operation. That is, the solid line B illustrated in FIG. 4 shows the relationship between the total heat exchange capacity of the first outdoor heat exchanger 10 and the second outdoor heat exchanger 20, and the airflow amount of the outdoor fan 6.

For example, the airflow amount of the outdoor fan 6 during the first cooling operation is assumed to be “C.” On this presumption, the heat exchange capacity of the condenser during the first cooling operation is “D.” The airflow amount of the outdoor fan 6 is “E” when the heat exchange capacity of the condenser during the second cooling operation is “D.” Thus, when the airflow amount of the outdoor fan 6 during the first cooling operation is “C,” the fan airflow-amount determination unit 63 determines that the airflow amount of the outdoor fan 6 during the second cooling operation is “E” in step S11. In Embodiment 1, in the manner as described above, the fan airflow-amount determination unit 63 determines the airflow amount of the outdoor fan 6 based on the relationship between the heat exchange capacity of the condenser and the airflow amount of the outdoor fan 6 illustrated in FIG. 4 in step S11. In the manner as described above, the airflow amount of the outdoor fan 6 is determined such that the heat exchange capacity of the condenser during the first cooling operation is equal to that during the second cooling operation. This can further reduce the variations in the temperature of air blown from the indoor unit 102 toward an air-conditioned space, compared to the case where the airflow amount of the outdoor fan 6 is simply reduced at the time of switching the first cooling operation to the second cooling operation. Note that the relationship between the heat exchange capacity of the condenser and the airflow amount of the outdoor fan 6 illustrated in FIG. 4 is stored as, for example, a map or a function in the storage unit 65.

As illustrated in FIG. 3, the air-conditioning apparatus 100 performs the operation in step S13 and step S14 in place of the operation in step S8 illustrated in FIG. 2. Specifically, when the comparative temperature is lower than the prescribed temperature in step S7, the fan airflow-amount determination unit 63 determines the airflow amount of the outdoor fan 6 at the time of switching the second cooling operation to the first cooling operation in step S13. More specifically, the fan airflow-amount determination unit 63 determines an increased airflow amount of the outdoor fan 6 relative to the present airflow amount of the outdoor fan 6. In Embodiment 1, the fan airflow-amount determination unit 63 determines the airflow amount of the outdoor fan 6, such that the heat exchange capacity of the condenser during the first cooling operation is equal to that during the second cooling operation, based on the relationship between the heat exchange capacity of the condenser and the airflow amount of the outdoor fan 6 illustrated in FIG. 4.

After step S13, the control unit 62 brings the valve 22 into the closed state in step 314 similarly to step S8 in FIG. 2 to switch the second cooling operation to the first cooling operation. In step S14, the control unit 62 brings the flow switching device 21 into the second flow passage state similarly to step S5 in FIG. 2. Further, in step 314, the control unit 62 increases the airflow amount of the outdoor fan 6. That is, the airflow amount of the outdoor fan 6 during the first cooling operation is increased relative to the airflow amount of the outdoor fan 6 during the second cooling operation. This can reduce the variations in the temperature of air blown from the indoor unit 102 toward an air-conditioned space when the second cooling operation is switched to the first cooling operation.

As described above, the air-conditioning apparatus 100 according to Embodiment 1 includes the compressor 1, the indoor heat exchanger 3, the first outdoor heat exchanger 10, the second outdoor heat exchanger 20, the valve 22, the temperature sensor 51, and the condensing-temperature detection device 30. The first outdoor heat exchanger 10 and the second outdoor heat exchanger 20 are connected in parallel between the compressor 1 and the indoor heat exchanger 3. The valve 22 is provided between the second outdoor heat exchanger 20 and the indoor heat exchanger 3. The valve 22 switches between an opened state and a closed state. In the opened state, the flow passage between the second outdoor heat exchanger 20 and the indoor heat exchanger 3 is opened. In the closed state, the flow passage between the second outdoor heat exchanger 20 and the indoor heat exchanger 3 is closed. The temperature sensor 51 detects a temperature of refrigerant that flows into the second outdoor heat exchanger 20 and liquefies. The condensing-temperature detection device 30 detects a condensing temperature of refrigerant. When the comparative temperature becomes equal to or higher than a prescribed temperature during the first cooling operation, the air-conditioning apparatus 100 according to Embodiment 1 switches from the first cooling operation to the second cooling operation. The first cooling operation refers to cooling operation in which the first outdoor heat exchanger 10 functions as a condenser, while the indoor heat exchanger 3 functions as an evaporator, and the valve 22 is in the closed state. The comparative temperature refers to a value obtained by subtracting the temperature detected by the temperature sensor 51 from the condensing temperature detected by the condensing-temperature detection device 30. The second cooling operation refers to cooling operation in which the valve 22 is brought into the opened state, compared to the state during the first cooling operation.

When refrigerant stagnates in the non-operating second outdoor heat exchanger 20 during the first cooling operation, the air-conditioning apparatus 100 according to Embodiment 1 switches the valve 22 from the closed state to the opened state, and thereby can return the refrigerant stagnating in the second outdoor heat exchanger 20 to the refrigerant circulating flow passage. Due to this operation, the air-conditioning apparatus 100 according to Embodiment 1 does not need to be provided with an additional bypass pipe through which the refrigerant stagnating in the non-operating second outdoor heat exchanger 20 can return to the refrigerant circulating flow passage. Therefore, even when the air-conditioning apparatus 100 according to Embodiment 1 is used to perform cooling operation under the condition of low outside air temperature, the increase in manufacturing costs can still be minimized compared to the related-art air-conditioning apparatus.

Embodiment 2

In Embodiment 1, an opening-closing valve is used as the valve 22. The valve 22 is not limited to this opening-closing valve. As illustrated in Embodiment 2, another valve may also be used as the valve 22, whose opening degree is selectable from among a plurality of levels in the opened state in which the flow passage between the second outdoor heat exchanger 20 and the indoor heat exchanger 3 is opened. Note that the items that are not particularly specified in Embodiment 2 are the same as those in Embodiment 1, and the same functions and constituents as those described in Embodiment 1 are denoted by the same reference numerals.

FIG. 5 is a refrigerant circuit diagram of an air-conditioning apparatus according to Embodiment 2.

The valve 22 according to Embodiment 2 has an opening degree that is selectable from among a plurality of levels in the opened state in which the flow passage between the second outdoor heat exchanger 20 and the indoor heat exchanger 3 is opened. More specifically, the valve 22 is a flow control valve whose opening degree is continuously variable. Note that the valve 22 may be a flow control valve whose opening degree is variable in stages. In the air-conditioning apparatus 100 according to Embodiment 2, similarly to the valve 22, the valve 12 also has an opening degree that is selectable from among a plurality of levels in the opened state in which the flow passage between the first outdoor heat exchanger 10 and the indoor heat exchanger 3 is opened. That is, the air-conditioning apparatus 100 according to Embodiment 2 can regulate the flow rate of refrigerant that flows through the first outdoor heat exchanger 10 by using the valve 12, and can regulate the flow rate of refrigerant that flows through the second outdoor heat exchanger 20 by using the valve 22. Due to this configuration, the air-conditioning apparatus 100 according to Embodiment 2 does not include the flow control valve 5 differently from Embodiment 1 in which the air-conditioning apparatus 100 includes the flow control valve 5.

The controller 60 in the air-conditioning apparatus 100 according to Embodiment 2 includes an opening-degree determination unit 64 as its functional unit. The opening-degree determination unit 64 is a functional unit to determine the opening degree of the valve 22.

FIG. 6 is a flowchart describing operation of the air-conditioning apparatus according to Embodiment 2 during a cooling period.

The air-conditioning apparatus 100 according to Embodiment 2 performs the operation in step S21 and step S22 in place of the operation in step S5 illustrated in FIG. 2. The control unit 62 brings the valve 22 into the opened state in step S22 similarly to step S5 in FIG. 2 to switch the first cooling operation to the second cooling operation. In step S22, the control unit 62 brings the flow switching device 21 into the first flow passage state similarly to step S5 in FIG. 2. With this operation, high-temperature and high-pressure refrigerant in gas form, discharged from the discharge port 1 a of the compressor 1, flows into the second outdoor heat exchanger 20. Then, the refrigerant stagnating in the second outdoor heat exchanger 20 is heated by the high-temperature and high-pressure refrigerant in gas form flowing into the second outdoor heat exchanger 20, and evaporates, and then flows out of the second outdoor heat exchanger 20. This allows the refrigerant stagnating in the second outdoor heat exchanger 20 to return to the refrigerant circulating flow passage.

In step S22, the control unit 62 opens the valve 22 to its opening degree determined by the opening-degree determination unit 64 in step S21, In step S21, the opening-degree determination unit 64 determines an increased opening degree of the valve 22 as the comparative temperature increases. The comparative temperature is a value obtained by subtracting the temperature detected by the temperature sensor 51 from the condensing temperature of refrigerant circulating in the refrigerant circuit. That is, the opening-degree determination unit 64 determines an increased opening degree of the valve 22 as the amount of refrigerant stagnating in the second outdoor heat exchanger 20 increases. Specifically, a state, in which the comparative temperature at the time of switching the first cooling operation to the second cooling operation is a first comparative temperature, is defined as a first state. In addition, a state, in which the comparative temperature at the time of switching the first cooling operation to the second cooling operation is a second comparative temperature higher than the first comparative temperature, is defined as a second state. When the first state and the second state are defined in this manner, the opening degree of the valve 22 at the time of performing the second cooling operation in the second state is increased relative to the opening degree of the valve 22 at the time of performing the second cooling operation in the first state.

As described above, when performing cooling operation under the condition of low outside air temperature, the air-conditioning apparatus 100 performs the first cooling operation to maintain the condensing temperature of refrigerant at a prescribed temperature or higher. That is, a comparison between the first cooling operation and the second cooling operation shows that when the air-conditioning apparatus 100 performs cooling operation under the condition of low outside air temperature, the air-conditioning apparatus 100 can operate more stably during the first cooling operation. In view of that, when focus is on the stable operation of the air-conditioning apparatus 100 under the condition of low outside air temperature, it is desirable for the air-conditioning apparatus 100 to end the second cooling operation for recovering the refrigerant stagnating in the second outdoor heat exchanger 20 and return to the first cooling operation as soon as possible. The refrigerant stagnating in the second outdoor heat exchanger 20 can be recovered more quickly as the opening degree of the valve 22 is increased. Thus, when focus is on the stable operation of the air-conditioning apparatus 100 under the condition of low outside air temperature, it is preferable that the opening degree of the valve 22 during the second cooling operation is set as large as possible.

On the other hand, as the opening degree of the valve 22 during the second cooling operation is increased, there is a greater difference in the heat exchange capacity of the condenser in the air-conditioning apparatus 100 between the first cooling operation and the second cooling operation. This results in more significant variations in the temperature of air blown from the indoor unit 102 toward an air-conditioned space. In view of that, when focus is on reducing the variations in the temperature of air blown from the indoor unit 102 toward an air-conditioned space, it is preferable that the opening degree of the valve 22 during the second cooling operation is set as small as possible.

Therefore, the air-conditioning apparatus 100 according to Embodiment 2 regulates the opening degree of the valve 22 in response to the amount of refrigerant stagnating in the second outdoor heat exchanger 20 during the second cooling operation as described above. The opening degree of the valve 22 during the second cooling operation is regulated in the manner as described in Embodiment 2. This allows the air-conditioning apparatus 100 to return to the first cooling operation quickly from the second cooling operation, and can also reduce the variations in the temperature of air blown from the indoor unit 102 toward an air-conditioned space.

Note that the air-conditioning apparatus 100 according to Embodiment 2 may also change the airflow amount of the outdoor fan 6 between the first cooling operation and the second cooling operation to regulate the heat exchange capacity of the condenser in the same manner as in Embodiment 1.

REFERENCE SIGNS LIST

1: compressor, 1 a: discharge port, 1 b: suction port, 2: check valve, 3: indoor heat exchanger, 4: flow control valve, 5: flow control valve, 6: outdoor fan, 7: accumulator, 8: opening-closing valve, 9: opening-closing valve, 10: first outdoor heat exchanger, 11: flow switching device, 12: valve, 20: second outdoor heat exchanger, 21: flow switching device, 22: valve, 30: condensing-temperature detection device, 31: pressure sensor, 41: subcooling heat exchanger, 42: bypass pipe, 43: flow control valve, 51: temperature sensor, 52: temperature sensor, 60: controller, 61: computation unit, 62: control unit, 63: fan airflow-amount determination unit, 64: opening-degree determination unit, 65: storage unit, 100: air-conditioning apparatus, 101: outdoor unit, 102: indoor unit 

1. An air-conditioning apparatus comprising: a compressor; an indoor heat exchanger; a first outdoor heat exchanger and a second outdoor heat exchanger connected in parallel between the compressor and the indoor heat exchanger; a valve provided between the second outdoor heat exchanger and the indoor heat exchanger to switch between an opened state in which a flow passage between the second outdoor heat exchanger and the indoor heat exchanger is opened, and a closed state in which the flow passage between the second outdoor heat exchanger and the indoor heat exchanger is closed; a temperature sensor configured to detect a temperature of refrigerant that flows into the second outdoor heat exchanger and liquefies; and a condensing-temperature detection device configured to detect a condensing temperature of refrigerant, wherein during first cooling operation in which the first outdoor heat exchanger functions as a condenser, the indoor heat exchanger functions as an evaporator, and the valve is in the closed state, when a comparative temperature becomes equal to or higher than a prescribed temperature, the air-conditioning apparatus switches to second cooling operation in which the valve is brought into the opened state, the comparative temperature being a value obtained by subtracting a temperature detected by the temperature sensor from the condensing temperature, and wherein the valve has an opening degree selectable from among a plurality of levels in the opened state, and when a state, in which the comparative temperature at a time of switching the first cooling operation to the second cooling operation is a first comparative temperature, is defined as a first state, and when a state, in which the comparative temperature at a time of switching the first cooling operation to the second cooling operation is a second comparative temperature higher than the first comparative temperature, is defined as a second state, an opening degree of the valve at a time of performing the second cooling operation in the second state is increased relative to an opening degree of the valve at a time of performing the second cooling operation in the first state.
 2. The air-conditioning apparatus of claim 1, comprising an outdoor fan configured to supply outside air to the first outdoor heat exchanger and the second outdoor heat exchanger, wherein an airflow amount of the outdoor fan during the second cooling operation is reduced relative to an airflow amount of the outdoor fan during the first cooling operation.
 3. (canceled) 